The present application generally relates to thermal management solutions, and more specifically to heat dissipation structures using aligned carbon nanotube arrays, and to methods of fabricating such a heat dissipation structure and applying it to a package.
With the development of microelectronic systems, for example, high brightness light emitting diode (HB-LED) for solid-state lighting, significant challenges of thermal management have to be faced to meet the increasing requirements of smaller profile, higher performance and longer product life time. More heat generated by devices needs to be effectively dissipated from a smaller area. Several kinds of heat sink are developed to expect to dissipate more heat from device to the environment. However it is very important to first conduct heat from device to heat sink by thermal interface materials. Unfortunately, conventional thermal interface materials, such as thermal grease thermal adhesives, phase change materials, etc., cannot meet the increasing requirement of the heat dissipation from a small area. Carbon nanotube (CNT) is an attractive candidate to improve the thermal performance of thermal interface materials because of their ultrahigh thermal conductivity up to 3000 W/m·K for multi-walled carbon nanotube (MWNT). Further information regarding CNT properties may be found in the Journal of the American Physical Society, Physical Review Letters, Vol. 87, page 215502 (2001), herein incorporated by reference. However thermal interface materials with randomly directed carbon nanotubes dispersed in epoxy resins or other matrix materials does not perform well because of the highly anisotropic nature of the thermal conduction by carbon nanotubes. Aligned carbon nanotube arrays directly extending from a first surface, for example a heat source surface, to a second surface, for example a cooler surface, is expected.
U.S. Pat. Nos. 6,965,513 and 6,924,335, incorporated by reference herein for all purposes, disclose thermal interface materials with carbon nanotube bundles embedded in matrix materials. However, the phonon heat transfer modes in matrix materials and carbon nanotubes are not compatible, which significantly limits the advantage of heat conduction by carbon nanotube. In addition, solidified matrix material is less flexible to fill in the uneven surfaces of heat source and heat sink. As a result, the thermal conductivity of thermal interface material with aligned carbon nanotube arrays in matrix is only 1.21 W/m·K and the contact thermal resistance is more than 50 mm2·K/W. Additional information regarding the thermal conductivity of thermal interface materials are detailed in Advanced Materials, Vol. 17, page 1652 (2005) incorporated by reference herein.
U.S. Pat. No. 6,856,016 and U.S. Patent Application Publication US 2004/0150100, both incorporated by reference herein for all purposes, disclose a thermal interface layer with carbon nanotubes grown on the surface of semiconductor die. However, the processes are not compatible for carbon nanotubes synthesis and device fabrication. If carbon nanotubes are grown before device fabrication, the decreased wafer cleanliness and ability to protect carbon nanotubes will make it difficult to conduct device fabrication using normal processes and equipments. Alternatively, if a device is fabricated before carbon nanotubes growth, the high temperature required by growing carbon nanotubes will damage the device or increasing the device cost by changing the processes and materials.
As for the connecting methods, U.S. Patent Application Publication U.S. 2004/0261987, incorporated by reference herein for all purposes, use an adhesion promoting layer to connect heat source and the array of carbon nanotubes. However, it is very difficult to form a very thin layer so that the tips of carbon nanotubes can still make contact with the heat source surface. As a result, there is actually another added layer with additional thermal resistance, which reduces the thermal performance of the thermal management solution. In U.S. Pat. No. 6,891,724, incorporated by reference herein for all purposes, carbon nanotubes grown from the opposed surfaces intermesh as the surfaces are mated. However, it is difficult for carbon nanotubes from any surface to extend directly to the other surface.